The invention relates to a method and apparatus for making in-situ measurements of the flow resistivity on the Earth's ground surface.
"Flow resistivity" is defined as the ratio of the pressure gradient across a test specimen to the volume of air flowing through the specimen per unit time. Because the flow resistivity is an important parameter appearing in many acoustical models of the Earth's ground surface, it is desirable to accompany measurements of acoustic ground impedance with measurements of flow resistivity.
Prior art resistivity instruments universally are comprised of four parts: an air supply or vacuum, one or more flow meters, one or more differential pressure measuring devices, and a specimen holder. The specifications on such instruments are given in ASTM Designation C522-80, "Standard Test Method for Airflow Resistance of Acoustical Materials."
The disadvantage of the prior art flow resistivity instruments is they are intrusive; that is, they require that a test specimen be removed from the bulk material for placement in a specimen holder. In the case of ordinary acoustical materials, which are generally chemically stable, removal of a specimen from the bulk does not affect its flow properties. In the case of the Earth's ground surface, however, removal of a specimen from the ground causes immediate and irreversible changes therein: loss of volatile matter, especially moisture; changes in texture; changes in organic content; and partial or total destruction of the root system of surface vegetation.
It is therefore the primary object of this invention to provide a method and apparatus for making in-situ measurements of the flow resistivity on the Earth's ground surface.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.